Rotary compressors provided with liquid injections are known in the art. These compressors can be for instance of the vane type or of the single or twin-screw type.
The liquid injected into the compressor can be for instance oil, water or condensate of the gas in the case of regrigeration compressors.
This liquid is coming from a high pressure source, the pressure of which is generated by the compressor itself. Said source may be a tank in which oil is separated from the compressed gas, by gravity; or may be the condenser in the case of a refrigeration unit.
The liquid is basically used to cool the compressor; in some cases, the liquid is oil, and also lubricates and seals the compressor. The position of the injection hole (or holes, for instance in a single screw machine provided with two opposed compression chambers and therefore with at least two symetrical injection holes) is important for the efficiency.
If the hole is located in the compressor casing near the low pressure area thereof, the efficiency is reduced; indeed, the liquid carries a lot of gas, this being particularly true in the refrigeration systems in which the condensate, coming from the condenser, is injected substantially at intake pressure and flashes.
The flash gas may reduce the amount of gas sucked by the compressor, that is to say reduce the capacity of the compressor. Moreover, the flash gas has to be recompressed and thus reduces the efficiency.
It is therefore advantageous to move the location of the injection hole towards the high pressure area of the casing so as to reduce the amount of work to recompress the flash gas. Moreover, if the injection hole is far enough from the low pressure area, each compression chamber will communicate with the injection hole only when said chamber no longer communicates with the low pressure area, and the flash gas will not be able to affect the amount of gas sucked by the compressor.
However, in compressors which are used with variable compression ratios between intake and discharge, such as refrigeration compressors, it happens that, sometimes, due to the conditions in the refrigerating circuit, the discharge pressure becomes substantially equal to or lower than the average pressure at the injection hole; in such a case, the liquid injection flow is stopped; the compressor is no longer cooled, and may be subjected to seizure.